Often it is necessary to couple a signal from one optical fiber to another. The couplings used, commonly known as splices, must be very precisely manufactured. This is to ensure that the fibers are accurately aligned so that light emerging from one fiber enters the other. In order to reduce the precision required, a tunable splice is sometimes used. In a tunable splice the alignment may be adjusted to ensure maximum signal transmission.
Another advantage of tunable splices lies in the ability to use such splices as variable attenuators. In a common situation, light from a light source, such as a laser diode, is transmitted by optical fibers to a detector. If the light received by the detector is too intense, the detector will go into saturation and be unable to accurately measure changes in intensity. Thus, information contained in the signal transmitted will be lost. This problem may be overcome with a tunable splice by slightly detuning it to attenuate the signal. Furthermore, the output of a semiconductor laser will commonly reduce over time as the laser ages. If this occurs the tunable splice may be adjusted to remove the attenuation and keep signal levels constant.
U.S. Pat. No. 3,800,388, issued to Manfred Borner et al. teaches a tunable splice wherein the fibers to be spliced are placed in holders, each of which is designed to be rotated around an axis. The axis of rotation of the holders are parallel but non-collinear. Each fiber runs through its respective holder in a direction parallel to but not on the axis of rotation thereof. By independently rotating the two holders the fibers may be aligned. After alignment the holders are secured in place. The process of securing the fibers in the Borner et al. splice can cause problems by partially dealigning the fibers.
U.S. Pat. No. 4,019,806, issued to Daniel Fellows et al. teaches the use of two cylindrical holders which are placed in a channel known as a V-groove. Each holder has an optical fiber running therethrough on an eccentric to the axis of the holder. One of the holders has its axis of rotation shifted with respect to the other through the use of a small shim which is placed along one wall of the V-groove. Such an approach reduces the likelihood of detuning when the holders are secured, but requires very precise manufacture of the holders and of the shim.
U.S. Pat. No. 4,239,333, issued to Mark L. Dakss et al. describes a fiber optic splice having a cylindrical plug for each fiber, each plug having a bore along its axis through which the respective fiber is inserted. These plugs are then inserted into sleeves in a pair of cylindrical supports. The sleeves are set eccentric to the supports so that the supports may be rotated to align the fibers. The axis of rotation of the two supports are adjusted to be different either by using a shim, as is done in Fellows et al., or by designing the supports to have different diameters. The system of Dakss et al. requires extremely close tolerances for the plugs and the sleeves in order to achieve and maintain alignment of the fibers.